Diamonds may be a girl's best friend, but exotic rainbow-hued stones are the must-have in bling this season according to German gem expert Alex Gerhard.

And if anyone knows about jewels, it's a man who travels the world with millions of dollars worth of rocks - and a bodyguard - in tow.

Gerhard, who cannot be photographed for security reasons, was in New Zealand last week to promote coloured gem stones, all of which seem to reduce even the most discerning bling lovers to magpie-like devotees.

"The New Zealand market is a very conservative market - people tend to buy the more traditional rubies, sapphires and emeralds, but I want to educate people that there are so many more options."

He said the most coveted stone this year is undoubtedly the neon blue Paraiba Tourmaline.

"It's one of the of the rarest stones in the world at the moment because of its unique colour," he said.

Paraiba Tourmaline stones have always been valued much higher than other tourmalines, but are now said to be worth up to USD$150,000 (NZ$186,000) per carat due to the low quantities available.

Fashion icon Victoria Beckham is the latest big name to invest in a Paraiba, which is sure to add to its covetability.

Nikki Partridge of Partridge Jewellers says jewellery trends tend to go in cycles, but coloured stones are definitely a big hit this year.

"People are having a lot more fun with vibrantly coloured pieces, and wearing them in big, standout cocktail rings and pendants," she said.

"Incorporating different colours into your jewellery collection means you can mix and match with your mood."

Another blue-coloured gem set to be a best-seller for Gerhard's gem-cutting business is natural tanzanite.

"The rough material of tazanite usually looks brown and has to be heated for the royal blue hue to come out, but one production came out of Tanzania this year where the colour was completely natural," he said.

Prices for gemstones like these are increasing tremendously, making them even more unattainable for the average consumer.

"Appropriate weather for mining, competition between different countries and expensive labour are all contributing to make the stones more precious than they have ever been," Gerhard said.